1. Field of the Invention
This invention relates to an ink-jet recording ink (hereinafter often simply “ink”) which is suited for ink-jet recording, does not cause any problem of the curling of recording mediums even when water-based inks are applied in a large quantity to cellulose-containing recording mediums, and affords good response and recording-head sticking resistance also when ejected at a high driving frequency.
2. Related Background Art
Ink-jet recording is a process in which small droplets of ink are made to fly to adhere to a recording medium such as paper to perform recording. According to a thermal ink-jet system in which an electricity-heat converter is used as an ejection energy feed means and heat energy is imparted to an ink to generate air bubbles to eject the droplets, its recording head can easily be made into a high-density multi-orifice head, and images with a high resolution and a high quality can be recorded at a high speed (Japanese Patent Publications Nos. S61-59911, S61-59912 and S61-59914).
Now, inks used in ink-jet recording are commonly those which are chiefly composed of water and, incorporated therein, coloring materials, and also water-soluble high-boiling point solvents such as glycols for the purpose of, e.g., preventing drying and improving recording-head sticking resistance. Where images are recorded on a recording medium containing cellulose as typified by plain paper, trace-coated paper or the like by using such inks, the ejecting of inks in a short time in a region of certain amount or more in area causes a problem that a phenomenon of curling (paper warps or rolls) may come. This problem has not come about in conventional recording performed chiefly for characters or letters, which has been prevalent. However, in recording in which inks are applied in a large quantity as in recording internet home page images or photographic images, such a problem is a great subject to be settled.
The droplets of inks ejected through a single ejection orifice have also been made small in size in order to deal with ink-jet recording images having a very high grade on the level of silver salt photography. At present, ink-jet printers of about 5 pl (picoliters) or less in ink droplet quantity have appeared on the market. Also, with regard to recording speed as well, printers have been demanded to be of much higher speed, and, as what is attendant thereon, it is a matter of great urgency to deal with higher driving frequency or improve recording-head sticking resistance.
In settling such subjects, water-based ink compositions containing various kinds of curling-preventive solvents are proposed (Japanese Patent Applications Laid-open Nos. H6-157955 and H11-12520). Although an effect can be seen to a certain extent in respect of curling resistance, a further improvement is demanded in respect of the simultaneous achievement of response in performing ejection at high driving frequency, recording-head sticking resistance, and curling resistance. Here, the respective subjects are outlined below.
1. Curling Resistance:
In applying water-based inks in a large quantity to recording mediums containing cellulose, as typified by plain paper, the phenomenon of what is called “curling” may occur and the paper rolls into cylindrical in some cases.
The mechanism by which the curling occurs is considered due to the fact that, in the step of drying paper in the stage of paper making, the water evaporates in the state a tension is applied to a certain direction to form hydrogen bonds between cellulose molecules. Upon adhesion of a water-based ink to the paper that is in such a state, the hydrogen bonds between cellulose molecules break because of the water and the bonded sites are substituted by the water, whereas, upon evaporation of the water, the hydrogen bonds are again formed between cellulose molecules. It is presumed that, when the hydrogen bonds are again formed, no tension is applied there and hence the paper shrinks on the side to which the ink has adhered, so that the curling occurs.
This phenomenon, which can not easily occur in the recording chiefly of characters or letters where the ink is applied in a small quantity, has come into a serious problem in these days where graphic printing is increasingly frequently performed, and it is demanded to improve curling resistance remarkably. This is of great demand especially in a condition where a water-based ink is ejected to a cellulose-containing recording medium of 15 cm2 or more in recording area and in an ink-application quantity ranging from 0.03 to 30 mg/cm2.
2. Frequency Response:
In an on-demand type ink-jet recording system, an attempt to eject inks continuously at a high driving frequency may make the inks not re-filled in channels in time depending on physical and chemical properties of the inks and inevitably begin to be ejected for the next before the channels are re-filled. As the result, this may cause faulty ejection or bring about a condition where the inks are ejected in a very small quantity. Also, this phenomenon occurs more conspicuously as the droplets of inks ejected are smaller.
3. Sticking Resistance (Resistance to Channel Clogging):
As another problem caused by the evaporation of water content of inks that takes place at channel ends, the clogging of channels may be given which occurs due to the sticking of coloring materials at channel ends because of the evaporation of water content; the sticking being caused, e.g., when a printer is left for a certain period of time without being used, or when, in a printer which is of a type its ink tank and printing head are set integral, the printer is left in the state the head itself is detached from the printer, or when, in a printer which is of a form its ink tank and printing head are set separable, the printer is left in the state the ink tank is detached from the printer.